This invention relates to hand tools, and in particular the invention is concerned with a compact hand tool for projecting a plumb and level beam of visible light for precise alignment. The device projects two beams simultaneously at 90 degrees to each other. When the tool is used in the upright position, the projected laser beams are level and plumb. When the tool is placed on its side in the horizontal plane the lines may be used to make a precise right angle as needed in construction applications. The leveling technique used combines the most attractive features of the existing leveling technology to produce an instrument with a wide self-leveling range, high accuracy over temperature, and low manufacturing tolerances. The unique method of leveling the level and plumb beams can be used with a mirrored cone to produce a plane of laser light for locating a plane in space.
A variety of survey and carpentry tools have previously employed lasers. The first laser alignment tools were manually leveled as in U.S. Pat. Nos. 3,897,637 and 3,279,070. Subsequently, self-leveling instruments were employed to improve accuracy and reliability. There are several methods of electronically leveling a platform on which the laser projector is mounted. Such systems use electronic level sensors to control motors which do the actual leveling. These systems are very expensive and complex.
Automatic self-leveling has been achieved using two distinctly different methods. In a first category of leveling instrument, the entire laser is leveled by gravity. In U.S. Pat. No. 3,771,876 a He-Ne laser and chassis are hung by a flexible support to create a plumb beam which is subsequently directed in the horizontal plane using a pentaprism. FIG. 1 herein shows that system, with the laser 4 hung by a wire 2 over a pentaprism 10. The pendulous oscillation is damped out by a magnet 12. The beam exits by a window 14, while the pentaprism 10 is rotated by a motor 16.
Another platform leveling system is described in U.S. Pat. No. 5,184,406 and is shown in FIG. 2a herein. A battery operated laser diode assembly 22 is mounted to a float 26 which is supported by a liquid 28 in a vessel 24. A collimated laser beam 20 remains plumb in spite of the tilt of the vessel.
A ball bearing pendulum of U.S. Pat. No. 5,144,487 is shown in FIG. 2b. A platform 23 is suspended by a ball bearing pivot 25 within a housing 29. An optical assembly within the platform generates multiple collimated laser beams 21 for alignment purposes. The platform motion is damped by eddy currents induced by a magnet 27. The use of several ball bearings makes the instrument expensive, large and of limited accuracy and ruggedness. At tilt angles close to level the force available to overcome the friction in the ball bearings is quite small, leading to inaccuracy.
In a second distinct category of laser alignment tools, the laser is rigidly mounted to the housing of the unit and a compensation means is used to correct for the tilt of the housing. In U.S. Pat. No. 3,684,381, as shown in FIG. 3 herein, a thin film of oil 36 is used to create a correcting prism which directs the downwardly directed laser beam 32 toward the plumb direction. The oil prism is formed by the upper level of the oil which is level and a lower window 38 which is tilted. The oil is contained by an upper window 34 and a housing 35. A laser source is mounted in a laser housing 30. This system is accurate only when two cells with oil of index of refraction 1.5000 are used. This system has proven inadequate because of the absence of a fluid with the desired properties over the normal temperature range. In addition the meniscus at the edges of the chamber contribute wave front errors. In the system of FIG. 3 a pentaprism 40 directs the plumb beam into the horizontal plane.
Several methods have been developed to tilt compensate a laser beam using wires. In U.S. Pat. No. 4,221,483 a pendulous lens hangs below a laser diode. As the housing is tilted the lens motion under gravity is proportional to the tilt angle which steers the laser beam to the plumb position. A pentaprism again converts the plumb beam to the horizontal plane. In U.S. Pat. Nos. 4,852,265 and 4,912,851 as shown in FIG. 4, a laser beam 41 is reflected from a mirror 43 on a platform 46 which is suspended from a single wire 44. The reflected laser beam 42 is compensated by the tilt of the platform under the force of gravity. The length and diameter of the wire are chosen so that when the housing of the unit is tilted exactly one degree, the platform tilts exactly one half of one degree. Because of the two-to-one relationship between mirror tilt and beam correction, the laser beam is restored to its plumb direction after being tilted. These patents also show laser diodes mounted on cantilevers which respond to housing tilt to correct the output beam.
A similar technique is used to produce a plane of light in U.S. Pat. No. 4,679,937 as shown in FIG. 5 herein. A plane of light can be created by reflecting a collimated beam of light from a mirrored cone as described in U.S. Pat. No. 4,111,564. The energy center of the beam is used to define the center of the optical reference plane. A collimated laser beam 54 is reflected into a plane 58 from the mirrored surface of a cone 56 suspended from a wire 60 which is supported by the housing 62. Magnets 64 provide the damping necessary for stable operation.
The laser platform leveling techniques of the first category suffer from accuracy limitations because as the platform approaches level the force available to correct for out of level decreases. In addition an expensive pentaprism is required to produce a level beam. In the wire hung platform of U.S. Pat. No. 3,771,876 (FIG. 1 herein) the error is dependent on the degree of out of level of the housing.
The wire beam techniques of the second category require a pentaprism to create a level beam or plane of light. It is a characteristic of reflection which results in inaccuracy of the 90 degree deviation when the normal vector to the mirror is out of the plane of the incident and reflected beams. As a result the pentaprism and mirrored cone reflectors have a self-leveling range of less than one half of one degree for accuracy better than 20 seconds. In addition these products bear the cost of these expensive reflectors.
An additional handicap of the instruments using tilt compensation is the need for tight tolerances in manufacturing the instruments. The requirement for a constant relationship between the platform tilt and the beam deflection defines the need for precision.